Known optical amplifiers using optical fibers for amplification as an optical amplification medium are such that the cores of the optical fibers are doped with a rare-earth element such as erbium or thulium. Of such amplifiers, an erbium-doped fiber amplifier (EDFA) using an erbium-doped optical fiber (EDF) as an optical amplification medium is used in an optical communication system, because it can amplify light having a wavelength of C-band (1530 nm to 1565 nm) that is used as a signal light wavelength range in optical communication.
In an wavelength division multiplex (WDM) optical communication system, an optical amplifier is used for collectively amplifying the multiplexed multi-wavelength signal light that is transmitted therethrough. In this case, it is desirable that the gain of optical amplification be flat (that is, the relative gain deviation is small) in a signal light wavelength range including the multiwavelength signal light.
FIG. 1 is a graph showing an example of a gain spectrum of an EDF. In general, the gain spectrum of an EDF has a peak near the wavelengths of 1530 nm and 1650 nm, respectively and has a bottom near the wavelength of 1535 nm. A relative gain deviation is defined by the expression:(Gmax−Gmin)/Gmin,where Gmax represents the maximal value among the peak values near the wavelength of 1530 nm and 1650 nm and Gmin represents the bottom value near the wavelength of 1535 nm. The smaller the relative gain deviation, the smaller the loss due to a gain equalizer can be made, and accordingly the amplification efficiency as a whole can be increased.
It is known that when the aluminum is co-doped into the core of an EDF, the relative gain deviation of the EDF becomes smaller. The literature by Kashiwada et al.: OAA'93, MA6, 104, (1993) discloses an EDF in which the aluminum is codoped at high concentration such that the relative gain deviation is largely improved.    Non-patent literature 1: T. Kashiwada et al.: OAA'93, MA6, 104, (1993)
However, there are cases in which aluminum is codoped in addition to erbium in the core of an EDF and another element such as germanium is further added to the core. When various kinds of elements are co-doped in the core, it is difficult to make EDFs stably by estimating the amplification performance from the composition of the EDF.